1. Field of the INvention
This invention generally relates to a head rest adapted for use on a vehicle seat back, and more particularly to an adjustable head rest device.
2. Statement of the Prior Art
Recently in the automobile industry, vertically movable head rest devices have been developed and one of them is disclosed in a Japanese Patent application published in 1985 as Jikkai-No. sho 60-167558. Such head rest, as shown in FIGS. 5 and 6, comprises stay 1001 embedded in a seat back 1000, guide shoe member 1002 slidably inserted into the stay, head rest body 1003 provided in the guide shoe and a lock mechanism 1005 for locking the sliding movement of the shoe member at selected portions. The lock mechanism of this type includes a plurality of detent teeth 1010 formed on the stay and an operating portion 1020 having detent member 1021 engageable with and disengageable from the detent teeth. Each of the detent teeth 1010 comprise flat portions 1010a and grooved portions 1010b.
In this prior art head rest device, a desired position of the head rest is obtained from the following steps:
adjusting the head rest body to a desired position relative to the seat back
operating the operation portion to engage the detent member with any of the detent teeth, thus locking the head rest body to the stay.
The above prior art head rest device, however, has a drawback in that it cannot carry out a fine adjustment due to the structure of the locking mechanism. As is shown in FIG. 6, when the detent member is on the flat portion of the detent teeth, the adjustment can only be performed by the length L (which corresponds to the length of a flat portion 1010a of one of the detent teeth 1010).